microSD card capacity

Index, feed.

I have those two Samsung microSD cards, nominally of the same model (MB-MC32D: 32GB Evo+ UHS-I, class 10).

Supposedly identical microSD cards

The one on the left on the picture has the following capacity in bytes:

# blockdev --getsize64 /dev/mmcblk0 
32026656768

The other card holds this many bytes:

# blockdev --getsize64 /dev/mmcblk0 
32010928128

That¹ is $2^{20} \times 3 \times 10181$ versus $2^{26} \times 3^{2} \times 53$, respectively, or, if we retain the largest common power of two factor only, $2^{20} \times 30543$ vs $2^{20} \times 30528$: the first card is larger by precisely 15 megabytes².

That capacity discrepancy was a bit unfortunate, as I intended to replace the card on the left, which was dying, and had recently turned itself read-only³, with the other one.

Follow-up: SanDisk

In late December, I finally received a new pair of replacement microSD cards (SanDisk Ultra microSDHC™ UHS-I "card with adapter, for better photos and full HD video, A1 for faster mobile app performance, 32GB, speed up to 98MB/s, 653x"; the front of the packaging bears a circled "class 10" logo, the back has SDHC I, microSDHC I, A1, and the card itself has that plus A1, microSDHC I, UHS-I, and the back of the packaging).

The card's capacity shows as:

# blockdev --getsize64 /dev/mmcblk0 
31914983424

This is $2^{19} \times 3 \times 103 \times 197$ bytes.

The newer SanDisk is smaller than the smallest of the two Samsung cards by 91½ megabytes.

www.kurokatta.org

David Haguenauer <www@kurokatta.org>
$Id: microsd-card-capacity.malblog,v 1.5 2021/01/04 04:03:01 homer Exp $